Historically the disposal of the sewage sludge residual solids at a biological wastewater treatment plant has been a significant operating expense. As a result, efforts have been made to minimize the volume through dewatering the residuals. Dewatering devices squeeze the water out of the semi-solids sewage sludge cake through pressure, over a period of time. Ten to twenty years ago dewatering would produce a cake to about 15-20% solids. Recent improvements in dewatering removes even more water, reducing transportation cost even further. A consequence of the drier solids is a greater resistance to the rate of flow when loading sewage sludge cake into trucks. Often the trucking is sub-contracted to private haulers. The longer it takes to load a truck the more expensive the cost of disposal becomes.
Typically, wastewater treatment plants dewater sewage sludge for several days in a row. During that time they will gradually fill up their sewage sludge storage bins over a period of a day to weeks. In some cases, the bins can hold as much as 50 cubic yards of material, which may weigh as much as 100,000 pounds. Therefore, compaction occurs over time. A natural consequence of storing a high volume of residual solids over an extended period of time is an increase in viscosity, because sewage sludge cake is a dilatant material. Therefore if the material is at rest and the weight is increasing every day the dewatered sewage sludge cake will become increasingly more difficult to move.
In sludge handling systems, sludge resides in storage bins until it is desired to discharge sludge from the bins. Generally, one or more bins are provided, and the sludge is delivered into the bins at upper ends thereof, and is discharged from the bins through a discharge opening at a lower end of each bin. Generally, the discharge opening is closed until it is desired to discharge from the bin, and at that point, an openable closure is activated, opening the discharge opening at the bottom of the bin, to allow sewage sludge in the bin to be discharged from the bin, generally through a chute to direct the sludge onto the bed of a waiting truck disposed therebeneath, with the sludge then being delivered to be used as agricultural ground cover, landfill, or the like, as may be desired.
Sometimes sludge resides in the bin for a day or more, such that it may become increasingly compacted, with sludge along side wall(s) of the bin becoming adhered to the side wall(s), such that when the discharge gate is opened, sludge may not readily flow outwardly of the bin, via gravity flow, also called mass flow, without some assistance. This can particularly occur, as the volume of sludge in the bin decreases, because the lower volume, or “head,” results in a reduced weight or pressure condition. As the “head” thus decreases, the resistance to flow increases, as sludge tends to adhere to the walls.
One approach that has been used to minimize aggregation of sludge at the bottom of a bin is to provide a rotationally driven discharge arm, that may be of plow-like construction, to sweep across a bottom floor of the bin, pushing sewage sludge along the bottom floor of the bin toward a central opening. Such a discharge arm will generally be motor-driven, either at a constant speed or a variable speed, and will undercut sewage sludge that may have become compacted during the time that it has been in the bin.
Such devices as these that move a discharge arm across a generally horizontal surface at the bottom of a bin, can be effective in many situations.
However, the longer a bin has been in use, the interior surfaces of the side wall(s) of the bin can become rough due to rusting, or due to other phenomena, such that sewage sludge may not slide easily along sloped surfaces of the bin, downwardly toward a discharge opening, but, rather, may accumulate on sloped surfaces of the bin.
Examples of sludge handling systems equipped with rotationally driven discharge arms are disclosed, for example, in U.S. Pat. Nos. 8,939,329 and 8,662,354.
As the sludge dewatering processes become more efficient, there is an increasing need to be able to process and dispose of the resulting sludge in an efficient manner.